Submarine cable systems originally were such as to connect two land terminals which were, for example, on opposite sides of the Atlantic ocean. A later development involved having two land terminals on one side and a third on the other side, there being a main cable extending between the third land terminal and a branching unit (y interconnect) and spur cables extending from the branching unit to the other two terminals. Repeaters may be disposed in the main cable and the spur cables and power feed provision must thus be made. The repeaters can be powered by supplying current between a land terminal (terminal station) at one end of a branch (main or spur) cable and a distant earth (single end feeding) or between any two terminal stations (double end feeding). Branching units have previously been proposed whereby power feed of repeaters in the main cable and one spur cable is by double end feeding, whereas that of repeaters in the other spur cable is by single end feeding, the earth being provided by a sea earth cable integral to the branching unit. The branching unit may include relays by means of which the power feeding can be changed (switched) in the event of fault conditions in one of the branches, in order to isolate that branch whilst continuing to power the other branches.
Optical fibre submarine cable systems are now being designed which have a number of branching units, that is they involve multiple landing points. One form of branching unit for use in such a system is described by M W Perry, G A Reinold and P A Yeisley in IEEE Journal on Selected Areas in Communications--Vol SAC-2 No. 6 November 1984. It has been found that under certain conditions, flashover within the relays of this arrangement can result in discharge of a main cable to earth. The consequent current usage can result in permanent damage to the system. A further branching unit suitable for use in multiple branching unit systems is disclosed in our GB published Application No. 2252686A (S M Webb 1) This known branching unit is a passive unit, i.e. it does not involve regenerators for the optical signals, designed to terminate three line cables and also provide a sea earth for power feeding.
It is an object of the present invention to provide an alternative form of branching unit suitable for use in multiple branch unit systems and which includes regenerators.